Our goals are to determine the mechanisms which determine cell type (mating type) in the unicellular eukaryote, the yeast Saccharomyces cerevisiae. In particular, we are studying the mechanism by which cells of one mating type switch to the other mating type and the mechanism by which the mating type locus controls cell type. The interconversion of mating types occurs by a process of genetic rearrangement: silent genetic blocks ("cassettes") become expressed when they are transposed to a "playback locus", the mating type locus. We propose to identify the genes necessary for this transposition process and to determine what roles they play in cassette transposition and to determine why cassettes are active at one locus and silent at others. We shall continue our studies of control of cell type by the mating type locus by cloning the genes hypothesized to be under control by the mating type locus and then by determining whether these genes are transcribed in various mutants defective in the mating type locus.